A direct-conversion, IQ receiver typically uses two mixers and two filters to down-convert a received signal into an in-phase component and a quadrature component. A first mixer mixes the received signal and an in-phase local oscillator signal to produce a first down-converted signal. The first down-converted signal is then processed by a first low-pass filter to remove unwanted high-frequency portions, leaving the desired in-phase component. A second mixer mixes the received signal and a quadrature-phase local oscillator signal to produce a second down-converted signal. The second down-converted signal is then processed by a second low-pass filter to remove unwanted high-frequency portions, leaving the desired quadrature component.
Under ideal conditions, the first mixer and the first low pass filter (collectively referred to as the in-phase path) supply an equal amount of gain and a difference in phase of 90 degrees to the in-phase component as the second mixer and second low pass filter (collectively referred to as the quadrature path) does to the quadrature component. In actual practice, factors such as manufacturing process non-idealities, temperature, and supply voltage cause a gain and phase imbalance between these two paths. These imbalances in a multicarrier communication system result in inter-carrier interference in the frequency-domain between each subcarrier of the received signal and its image, which degrades the performance of the receiver.
Therefore, the gain and phase imbalance between the in-phase path and the quadrature path are typically calibrated, i.e., estimated and compensated for, before the receiver begins receiving data (e.g., at power up). However, further gain and phase imbalances between the in-phase path and the quadrature path can occur after the receiver begins to receive data due to intentional or unintentional changes in the receiver. The conventional approach to dealing with these further gain and phase imbalances is to reset the receiver and re-perform the estimation and compensation process. Although effective in calibrating the paths, such an approach reduces the potential throughput of the receiver.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.